Silver halide photography usually involves the exposure of silver halide photographic element with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. In order to sensitize the silver halide to other than the blue region, sensitizing dyes are used in the silver halide emulsion. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds). Their usual function is to adsorb to the silver halide and to absorb light (usually other than blue light) and transfer that energy via an electron to the silver halide grain thus, rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. However, sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum.
Sensitizing dyes are typically selected which provide a high sensitivity to the emulsion in the wavelength region of interest. An increased sensitivity of an emulsion without increasing grain size also allows for an improvement in sharpness and/or a lowering of graininess. Higher sensitivities can also allow higher color saturation. Usually the sensitizing dyes are also selected such that the emulsion has accurate spectral response to enable the building of films having correct color reproduction. For example, a photographic element containing a green sensitized emulsion which has a peak sensitivity around 555 nm, will tend to reproduce red and orange objects with a magenta contamination due to lack of long green sensitivity.
In building photographic elements, it is also known to use strongly adsorbing additives, as stabilizers or antifoggants. These include azoles, mercaptocompounds, thioketocompounds, and azaindenes. In particular a 4-hydroxy substituted (1,3,3a,7)-tetraazaindene, such as 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene ("TAI"), is often used as a stabilizer. Such additives, particularly TAI, often desensitize emulsions apparently by desorbing sensitizing dyes from the emulsion grain surfaces. Thus, an emulsion which might otherwise have high sensitivity throughout the wavelength region of interest to provide correct color reproduction, may lose speed when a large amount of a strongly adsorbing additive (particularly TAI) is added.
Silver halide sensitizing dyes are of the cyanine type are well known. For example, U.S. Pat. No. 4,362,813 discloses combinations of bis-benzoxazoles and oxathiazole type dyes. However, the dyes described exclude bis-napthoxazole types. U.S. Pat. No. 4,594,317 generally describes combinations of three dyes. The patent specifically indicates that bis-napthoxazole type dyes are non-preferred. Other multiple dye combinations are disclosed, for example, in U.S. Pat. Nos. 5,041,366 and 4,571,380.
It would be desirable then, to provide a photographic element which is sensitized in the green region by dyes which provide a high maximum sensitivity, as well as preferably having good long green sensitivity, and which are affected to a lower extent by additives such as TAI.